Printing Devices

Types of Printers

Regardless of the size of the system, quantity of printing, or capacity of the printer, modern printers use one of three technologies to print dots: dot matrix, inkjet, or laser.

Dot matrix technology results from physical impact of the print head onto paper. The print head on a dot matrix printer consists of a number of printing pins, usually between seven and twenty-four, whose positions can be controlled by individual electromagnets. When a current is applied, the corresponding pin is forced to strike the paper through an inked ribbon to form a dot. Using more pins and overlapping dots by multiple passes over the same line can increase print quality. Most dot matrix printers can operate in several modes offering different tradeoffs between print quality and speed.

In recent years, better print heads and more intelligent controllers have improved the size and accuracy of the dots so that dot matrix printers, originally only intended for character printing, can now print high-quality graphics with resolutions that rival laser printers. Multiple inked ribbons can be used to produce color. However, dot matrix color is not considered acceptable for most purposes. Dot matrix printers are inexpensive in terms of ink ribbon and are reliable, but they are slow, noisy, and poor at graphics. Dot matrix printers have three major uses: printing on large preprinted forms; printing on small pieces of paper (such as cash register receipts, ATM machine receipts, credit card transactions, and airline tickets); and multi-part, continuous forms with carbon paper in between.

Inkjet printers use nonimpact printing by spraying heated ink from a tiny nozzle onto paper. The tiny nozzle is smaller than the width of a human hair and the volume of each sprayed droplet is about one-millionth the volume of an eye-drop. Inside each nozzle, an ink droplet is electrically heated to its boiling point until it explodes so that the only place the ink can go is out the front of the nozzle. The nozzle is then cooled and the resulting vacuum sucks in another droplet of ink. The speed of the printer is thus limited by the length of the boil/cool cycle.

Inkjets have resolutions ranging from 300 dpi to 1440 dpi. Some inkjet printers use a vibrating piezo-crystal instead of heat to produce ink droplets. In ink jet printers, the electric current causes a deformation of the crystal that squeezes out the drop of ink. Mechanically the inkjet printer works similarly to a dot matrix printer. It moves across a page to print a row, and mechanical rollers move the paper downward to print successive rows. Multiple reservoirs of ink may be available to print multiple colors. Inkjet printers are popular for low-cost home printing because they are small and economical. However, they are slow and produce ink-soaked output that can be messy.

Laser printing is derived from xerography with the difference that the image is produced electronically from the computer using a laser or light-emitting diode rather than scanned with a bright light. There are four steps in the operation of a laser printer. First, a laser illuminates the dots to be printed on a photosensitive drum that becomes electrically charged wherever a dot is to be printed. Second, the photosensitive drum rotates the charged dots into a tank of black toner where the toner sticks to the drum wherever a charge is present. Third, a sheet of paper is fed toward the drum, coated with electrical charges, and contacts the drum, picking up the toner image from the drum. Finally, the toner image on the paper is heated in a fusing system that melts the toner into the paper while the drum has its charge erased by the corona wire in preparation for the next page.

Other Printing Devices

The complete set of colors that a printer can produce is called its gamut. No printer has a gamut that matches the real world. It is limited by colors, discrete intensities, imperfections, non-uniform spacing across the color spectrum, and human perception. Transferring a color image that looks perfect on a computer display to an identical printed page is difficult for the following reasons:

• Display monitors use transmitted light, while color printers use reflected light;
• Display monitors produce 256 intensities per color, while color printers use half-tones;
• Display monitors have a dark background, paper has a light background;
• Display monitors have a RGB (red, green, blue) gamut and printers have a CYMK (cyan, yellow, magenta, black) gamut.

Common ways to print color images include the use of special inks and paper. Inkjet printers can use dye-based inks or pigment-based inks. Dye-based inks consist of colored dyes dissolved in a fluid carrier. They provide bright colors and flow easily. Pigment-based inks contain solid particles of pigment suspended in a fluid carrier that evaporates from the paper leaving the pigment behind. They do not fade in time like dye-based inks, but they are also not as bright, and the pigment particles tend to clog nozzles requiring periodic cleaning. Coated or glossy paper, specially designed to hold ink droplets and not let them spread, is required for printing photographs.

Solid ink printers are slightly higher quality than inkjet printers. These printers accept solid blocks of special waxy ink that is then melted in hot ink reservoirs. Startup times of these printers are long (typically ten minutes) while the ink blocks are melting. The hot ink is sprayed onto the paper, where it solidifies and is fused with the paper by forcing it between two hard rollers.

A step up from solid ink printers is a color laser printer. A color laser printer works exactly like a black and white laser printer, except an image is transferred to the roller using four different toners (the CYMK gamut). Because the full bit map of an image is generally produced in advance, the memory requirements make this type of printer expensive, but printing is fast, high quality, and images are stable over time. A 1200 by 1200 dpi image for a page containing 516 square centimeters or 80 square inches needs 115 million pixels. At 4 bits per pixel, the printer needs 55 MB just for the bit map, exclusive of memory for the internal processors and fonts.

For highest-quality color images, more specialized methods such as thermal wax transfer or dye sublimination are required. Sublimation is the scientific name for a solid changing into a gas without passing through a liquid state. The mechanisms for both methods are similar. The paper is fed into the printer and clamped against a drum with a print head providing a row of dot-sized heating elements. Between the paper and the print head, a roll of impregnated film is exposed. The heat from the print head melts the wax or dye onto the paper. The film is impregnated with either colored wax or dye in page-sized sections of cyan, yellow, magenta, and black.

For thermal wax transfer, the input paper is pre-coated with clear wax to compensate for slight paper imperfections and so that the wax may be applied more uniformly. For dye sublimation, the dyes diffuse and actually blend in the paper. Although dye sublimation can print continuous color tones without half toning, it also requires high temperatures and expensive specialized paper. The colors fade when exposed to ultraviolet light, such as that contained in sunlight. Small snapshot printers often use the dye sublimation process to produce highly realistic photographic images.

Organizations that produce and store significant copies of paper documents often use computer output microfilm (COM) devices to place data from a computer directly onto microfilm, thus eliminating the need for photographic conversion.

Evolution of Printing and Plotters

Early printers were derived from typewriters using daisywheel printers— form characters that were mounted at the ends of arms attached to wheels shaped like a daisy. Like typewriters, printing resulted from the wheel rotating to the proper position and an energized magnet forcing the wheel through an inked ribbon onto paper. These printers were difficult to maintain and incapable of generating any graphical images or foreign words that could not be formed by the given set of formed characters. Most formed-character impact printers have disappeared from use. Nearly all modern computer printers produce their output as a combination of dots.

Plotters are hard-copy printing devices consisting of one or multiple independently maneuverable ink pens that are used for general design work such as blueprints, schematics, drawings, and plotting mathematical functions. Standard plot widths are 61 centimeters by 91.5 centimeters or 24 inches by 36 inches, but the attractive feature of plotters is limitless size when necessary for uses such as graphic arts.

SEE ALSO INPUT DEVICES; POINTING DEVICES; SERIAL AND PARALLEL TRANSMISSION.

William J. Yurcik

Bibliography

Chambers, Mark L. Printer Handbook, 2nd ed. Foster City, CA: IDG Books Worldwide, 2000.